EMQX moves MQTT messages between devices; Cloud Studio IoT is what reads them, stores them and acts on them. Every instance of the platform has its own MQTT server, with TLS on port 8883 and its own credentials, so devices can publish to it directly or EMQX can bridge the topics towards it. Nothing in your broker has to change.
In Gear Manager, under Security > MQTT Configuration, create the user and password. That user id is also the prefix of every topic.
Connect the devices, or an EMQX bridge, to your instance's MQTT server over TLS on port 8883.
Each device publishes to {MQTTUserID}/uplink/{DeviceAddress}. Commands travel back to the device over its downlink topic.
Natively supported models are decoded out of the box. For your own payloads, a JavaScript script turns the message into the variables you want to keep.